1. Field of the Invention
This invention relates to absorbent film provided by a vacuum evaporation technique.
2. Description of the Prior Art
It has widely been practiced to produce absorbent film by the use of the vacuum evaporation technique and employing such film for color filters, neutral filters, sun glasses and phase plates for the differential phase technique.
Such absorbent film is usually composed of a metal or a metal compound especially such as metal oxide, and has a high refractive factor and a high absorption coefficient, which will not only result in a high reflection factor in both sides of the film adjacent the atmosphere and the base glass but also lead to a less aesthetic appearance and a great inconvenience in use.
In order to reduce the amount of reflected light, a single or multiple layers of dielectric film have been formed on the absorbent film by using the vacuum evaporation technique so as to prevent the occurrence of light reflection. Even this method, however, has encountered difficulties in effectively preventing the reflection of visible rays over their entire range, and the reflected light in this case is always seriously colored. To attain a predetermined coloring of such reflected light, the layer or layers of dielectric film to be formed on the absorbent film must be of a very high accuracy and otherwise controlled, and this makes it difficult to achieve a reproductivity of such film. In addition, presence of any slight fingerprint, water drop, fatty or greasy material or the like on the film would immediately vary the coloring of light to seriously injure the aesthetic value of the film.
Evidently, these drawbacks may be avoided by providing a thick and uniform film of a low absorption coefficient and whose refractive factor for the entire range of visible light is about 1.52, which is approximately equal to that of the conventionally used base glass. This is because such a film formed on the glass surface would permit only about 4% of the incident light to be reflected and thereby eliminate any coloring. The formation of such film may be accomplished by any of the various methods shown below.
I. A mixture of a dielectric material (non-absorbent material) and a metal (absorbent material) is evaporated from a single source of evaporation;
ii. The same mixture is evaporated from two discrete sources of evaporation; or
iii. A non-uniform film is produced so that the density ratio of the absorbent material to the non-absorbent material is least in the opposite surfaces of the film adjacent to the base glass and the atmosphere and greatest in the intermediate region of the film. The refractive factor in the said opposite surfaces of the film is substantially equal to the refractive factor of the base glass.
Success of the first-named method above is very much dependent on the materials selected, and usually it is very difficult to form a uniform and sufficiently thick film of a mixture at a predetermined ratio through evaporation. Even if such film could be attained at all, any thickness thereof exceeding a certain value would cause the film to readily separate from the base glass (weak adhesion) or to be fractured (inferior mechanical strength).
In some instances such film is poor in the resistivity to abrasion, heat and humidity, and in many instances it becomes seriously brownish due to the oxidation resulting from ultraviolet rays. A film having a high resistivity to mechanical, chemical and weather conditions could be formed by this method, but this would be impossible without resorting to the electronic beam evaporation technique or the like.
The second-named method above is unsuitable for mass production because it involves very delicate control of the rates of evaporation from the two sources of evaporation and some other delicate controls in order to provide a uniform film of a predetermined mixing ratio.
The third-named method is also difficult in terms of reproductivity because considerably complex and sophisticated controls are involved therein as in the case of the second method.